1. Field of the Invention
The present invention relates to the design of Ethernet. More specifically, the present invention relates to a method and an apparatus for delineating data in an FEC-coded Ethernet frame.
2. Related Art
In order to keep pace with increasing Internet traffic, optical fibers and associated optical transmission equipment have been widely deployed to substantially increase the capacity of backbone networks. However, this increase in the capacity of backbone networks has not been matched by a corresponding increase in the capacity of access networks. Even with broadband solutions, such as digital subscriber line (DSL) and cable modem (CM), the limited bandwidth offered by current access networks creates a severe bottleneck in delivering high bandwidth to end users.
Among the different technologies presently under development, the Ethernet passive optical network (EPON) is one of the best candidates for next-generation access networks. EPONs combine ubiquitous Ethernet technology with inexpensive passive optics. Hence, they offer the simplicity and scalability of Ethernet with the cost-efficiency and high capacity of passive optics. In particular, due to the high bandwidth of optical fibers, EPONs are capable of accommodating broadband voice, data, and video traffic simultaneously. Such integrated service is difficult to provide with DSL or CM technology. Furthermore, EPONs are more suitable for Internet Protocol (IP) traffic, since Ethernet frames can directly encapsulate native IP packets with different sizes, whereas ATM passive optical networks (APONs) use fixed-size ATM cells and consequently require packet fragmentation and reassembly.
Typically, EPONs are used in the “first mile” of the network, which provides connectivity between the service provider's central offices and business or residential subscribers. Logically, the first mile is a point-to-multipoint network, with a central office servicing a number of subscribers. A tree topology can be used in an EPON, wherein one fiber couples the central office to a passive optical splitter, which divides and distributes downstream optical signals to subscribers and combines upstream optical signals from subscribers.
Using EPONs in the first mile, however, is not without limitations. Because EPONs adopt the passive optical transmission technology, which does not involve amplification or regeneration, the size of a network is subject to power budget and various transmission impairments. Consequently, as a network increases its size, the signal-to-noise ratio suffers, resulting in more frequent bit errors. Fortunately, forward error correction (FEC) can mitigate these undesirable effects and can help increase the power budget.
FEC is an error correction technique wherein a receiving device has the capability to detect and correct any block of symbols that contain fewer than a predetermined number of error symbols. A transmitting device accomplishes FEC by adding bits to each transmitted symbol block, using a predetermined error correction technique. One commonly used technique is to use a Reed-Solomon code. A Reed-Solomon code is specified as RS(l, k) with s-bit symbols, which means that the encoder takes k data symbols of s bits each, and adds (l−k) parity symbols to make an l-symbol codeword. A Reed-Solomon decoder can correct up to t symbols that contain errors in a codeword, where 2t=l−k. For example, RS(255, 239) with 8-bit symbols means that each codeword contains 255 bytes, of which 239 bytes are data and 8 bytes are parity. The decoder can automatically correct errors contained in up to 8 bytes anywhere in the codeword.
Because FEC coding provides robustness against bit errors, FEC-coded Ethernet frames can survive harsh transmission environment which conventional Ethernet frames may not survive. However, one concern in implementing FEC-coded Ethernet frames is that they should be backward-compatible. That is, non-FEC-enabled equipment should be able to recognize FEC-coded Ethernet frames. For this reason, as proposed in the current IEEE 802.3ah Ethernet in the First Mile standard (hereinafter “IEEE 802.3ah standard), the FEC parity bits for all the blocks of data symbols are aggregated and appended to a conventional Ethernet frame. A delimiter that can be recognized by non-FEC-enabled equipment delineates the conventional Ethernet frame from the parity bits.
Unfortunately, this delimiter is not part of the FEC code and hence is not protected against bit errors. Bit errors that occur within the delimiter may confuse the receiving device, resulting in a truncated or corrupt Ethernet frame. Hence, what is needed is a method and an apparatus for delineating data in an FEC-coded Ethernet frame which is robust against bit errors within the delimiter.